When one wishes to attach plastic parts together, several processes exist to accomplish this. Among them are methods such as hot air bonding, sonic welding, vibration welding, adhesives, mechanical fasteners and infrared radiant heat.
U.S. Pat. No. 5,750,470 discloses a method of dielectrically heating an adhesive which, in turn, bonds plastic parts together.
Infrared radiant heat has certain advantages over the other types of processes. Generally, there is a desire to form a strong bond between the two parts quickly at a minimal cost. Additionally, it is preferable that during the bonding process nothing touches the surfaces to be bonded in order to assure uniform bonds from part-to-part and to reduce the cost of cleaning the bonding apparatus. Further, in many instances, it is preferable that no marring or distortion occur on the surface opposite the surface which is bonded. This surface is referred to as the "viewing" or "class A" surface.
Infrared bonding, in general, can overcome many of these concerns. Infrared energy can be finitely pinpointed with a focal point or a mask to the exact area to be bonded in order to avoid overheating the plastic in adjacent areas that might cause unwanted distortion in the part. Infrared energy can heat the bonding surface to a high temperature, thus assuring that a strong bond is formed between the two parts. The bonding surface can be heated very quickly with infrared energy and the timing and amount of heat application can be precisely controlled. Further, with infrared heating, there need be no contact between the heat source and the bonding surfaces of the parts, in order to minimize cleaning requirements for the tooling.
Nonetheless, there are some drawbacks, in general, to using infrared heat to bond plastic parts together. In the case of automotive interior parts, for example, there are many parts made of plastic that are bonded to another part, but also need to have their viewing surfaces as distortion and mar free as possible since occupants of the vehicle will be able to see the surfaces. However, in the general application of focused infrared heat on the bonding surface in such applications, the "class A" surface is oftentimes damaged. The finite focal point, while avoiding overheating adjacent areas, is too intense and harms many plastic parts during the heating process. An inherent difficulty to overcome in working with infrared radiant heat to bond plastic is to heat the bonding surfaces sufficiently without causing warpage, burning or marring of the "class A" surfaces.
U.S. Pat. Nos. 3,383,265; 3,549,451; 4,265,954; 5,151,149; 5,244,525; 5,444,814 and 5,522,954 all disclose the use of infrared energy to bond or join plastic or polymeric materials together. The '954 patent discloses the bonding of plastic interior automatic parts having "class A" surfaces after each surface is exposed to infrared heat from an infrared heat source positioned between the surfaces.
U.S. Pat. No. 4,096,306 discloses a method of forming air-inflated cushioning material using two heat sealable films with air passages therebetween, which may be fused by "infrared black line sealing."
One prior art bonding process used hot air to activate a relatively expensive heat-activated adhesive. However, the use of hot air: 1) has a relatively long cycle time; 2) requires a relatively expensive heat-activated adhesive; and 3) requires a relatively large initial capital investment which equipment is relatively expensive to maintain.
The use of heat-activated adhesive to bond parts together is shown by the following U.S. Pat. Nos. 5,228,108; 5,743,982; 5,486,252; 5,858,159; and 5,743,408.